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  <div class="section" id="the-pdb-serialized-hash-table-format">
<h1>The PDB Serialized Hash Table Format<a class="headerlink" href="#the-pdb-serialized-hash-table-format" title="Permalink to this headline">¶</a></h1>
</div>
<div class="section" id="introduction">
<span id="hash-intro"></span><h1>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h1>
<p>One of the design goals of the PDB format is to provide accelerated access to
debug information, and for this reason there are several occasions where hash
tables are serialized and embedded directly to the file, rather than requiring
a consumer to read a list of values and reconstruct the hash table on the fly.</p>
<p>The serialization format supports hash tables of arbitrarily large size and
capacity, as well as value types and hash functions.  The only supported key
value type is a uint32.  The only requirement is that the producer and consumer
agree on the hash function.  As such, the hash function can is not discussed
further in this document, it is assumed that for a particular instance of a PDB
file hash table, the appropriate hash function is being used.</p>
</div>
<div class="section" id="on-disk-format">
<h1>On-Disk Format<a class="headerlink" href="#on-disk-format" title="Permalink to this headline">¶</a></h1>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>.--------------------.-- +0
|        Size        |
.--------------------.-- +4
|      Capacity      |
.--------------------.-- +8
| Present Bit Vector |
.--------------------.-- +N
| Deleted Bit Vector |
.--------------------.-- +M                  ─╮
|        Key         |                        │
.--------------------.-- +M+4                 │
|       Value        |                        │
.--------------------.-- +M+4+sizeof(Value)   │
         ...                                  ├─ |Capacity| Bucket entries
.--------------------.                        │
|        Key         |                        │
.--------------------.                        │
|       Value        |                        │
.--------------------.                       ─╯
</pre></div>
</div>
<ul class="simple">
<li><p><strong>Size</strong> - The number of values contained in the hash table.</p></li>
<li><p><strong>Capacity</strong> - The number of buckets in the hash table.  Producers should
maintain a load factor of no greater than <code class="docutils literal notranslate"><span class="pre">2/3*Capacity+1</span></code>.</p></li>
<li><p><strong>Present Bit Vector</strong> - A serialized bit vector which contains information
about which buckets have valid values.  If the bucket has a value, the
corresponding bit will be set, and if the bucket doesn’t have a value (either
because the bucket is empty or because the value is a tombstone value) the bit
will be unset.</p></li>
<li><p><strong>Deleted Bit Vector</strong> - A serialized bit vector which contains information
about which buckets have tombstone values.  If the entry in this bucket is
deleted, the bit will be set, otherwise it will be unset.</p></li>
<li><p><strong>Keys and Values</strong> - A list of <code class="docutils literal notranslate"><span class="pre">Capacity</span></code> hash buckets, where the first
entry is the key (always a uint32), and the second entry is the value.  The
state of each bucket (valid, empty, deleted) can be determined by examining
the present and deleted bit vectors.</p></li>
</ul>
</div>
<div class="section" id="present-and-deleted-bit-vectors">
<span id="hash-bit-vectors"></span><h1>Present and Deleted Bit Vectors<a class="headerlink" href="#present-and-deleted-bit-vectors" title="Permalink to this headline">¶</a></h1>
<p>The bit vectors indicating the status of each bucket are serialized as follows:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>.--------------------.-- +0
|     Word Count     |
.--------------------.-- +4
|        Word_0      |        ─╮
.--------------------.-- +8    │
|        Word_1      |         │
.--------------------.-- +12   ├─ |Word Count| values
         ...                   │
.--------------------.         │
|       Word_N       |         │
.--------------------.        ─╯
</pre></div>
</div>
<p>The words, when viewed as a contiguous block of bytes, represent a bit vector
with the following layout:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>  .------------.         .------------.------------.
  |   Word_N   |   ...   |   Word_1   |   Word_0   |
  .------------.         .------------.------------.
  |            |         |            |            |
+N*32      +(N-1)*32    +64          +32          +0
</pre></div>
</div>
<p>where the k’th bit of this bit vector represents the status of the k’th bucket
in the hash table.</p>
</div>


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